Power mains supply unit for directly and/or indirectly supplying consumers with power

ABSTRACT

A mains power supply unit for the purpose of supplying consumers that are connected directly or by way of a voltage system converter to a direct voltage branch of a mains power supply unit comprises a buffer capacitor (C) that can be charged from the mains supply and that still supplies the direct voltage branch for a predetermined minimum period of time in the event of a failure or interruption in the mains supply. In such an event, the buffer capacitor (C) can be charged by way of a dedicated charging circuit ( 3 ) to a voltage (U 2 ), which, regardless of the voltage fluctuations in the direct voltage branch, is substantially constant and corresponds in a substantial manner to the upper limit value of the direct voltage (U 1 ) to which limit value the mains power supply unit is dimensioned. The buffer capacitor can be connected onto the direct voltage branch by way of a controlled switch (S) in the event of undervoltage in the mains supply or in the direct voltage branch.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a mains power supply unit for the purpose ofsupplying consumers that are connected directly or by way of a voltagesystem converter to a direct voltage branch of the mains power supplyunit, comprising a buffer capacitor that can be charged from the mainssupply and still supplies the direct voltage branch for a predeterminedminimum period of time in the event of a failure or interruption in themains supply.

2. Description of the Prior Art

In the case of electric energy converters, buffer capacitors are mostlyprovided, for example, in switched mode mains power supply units havingan intermediate circuit voltage. In the case of a large mains supplyvoltage range, buffer capacitors of this type are to be dimensioned tothe maximum voltage. Consideration must be given, however, to the factthat when there is a failure in the mains supply, the capacitors areonly charged to the intermediate circuit voltage corresponding to theminimum mains supply voltage. Owing to the quadratic dependency of theenergy content of a capacitor upon the voltage, the stored energy iswell below the possible maximum, so that the buffer time becomes tooshort, for example, to make possible any further procedures of dataprotection for the consumer.

Regardless of the problem of the energy content, when supplying fromalternating current mains supply, extreme distortions and correspondinghigh harmonic content in the alternating current mains supply may occurby virtue of the periodic, pulsed recharging of the capacitors.

Three different solutions are known for eradicating or reducing theseproblems:

a) Broaden the working range of the consumer (converter) to achievelower supply voltages, whereby the buffer capacitors can be dischargedto a lower voltage when the function of the consumer is maintained. Thedisadvantages of this solution are high development costs, as thisalmost reaches the boundaries of technical viability, inefficient use ofthe converter and that the distortions are not taken into consideration.

b) With regard to the required energy content, dimension the buffercapacitor to the minimum supply voltage. This results in the buffercapacitor being considerably over-dimensioned and also results in only apart of the storage capacity being used. Weight, volume and price of thebuffer capacitor take on undesired values and the distortions to themains supply become even greater.

c) By preconnecting a stabilised high-setting step, hold the voltage atthe buffer capacitor at a fixed value irrespective of fluctuations inthe mains supply voltage. This solution requires a dedicated switchedmode mains power supply step, and solves the two problems stated above.However, it also demands a considerable outlay for the development andproduction, and to accept a larger space requirement.

EP 0 525 898 discloses a circuit that, by way of a capacitive voltagedivider, obtains a feed current for the control circuit of a controlledswitch, in order to enable the circuit to run-up during initiation ofsaid circuit. Neither a buffer capacitor nor the controlled connectionthereof is disclosed.

EP 0 622 889 A2 describes a switched mode mains power supply unit, whichis to serve as a pre-controller, for example, for televisionapparatuses, and wherein the direct current at a load capacitor iscontrolled by means of an FET switching transistor so that this voltagesis always substantially equal in the event of different alternatingmains supply voltages, for example, 240 V and 120 V. The load capacitoris always connected in parallel with the load and cannot be consideredto be a buffer capacitor.

The circuit disclosed in EP 0 223 316 A2 is a high-setting controllerthat has variable switching frequency and comprises a load capacitorthat is always connected in parallel with the load, a capacitor of an LCinput filter and a capacitor of an RC integration element, but not,however, a connectable buffer capacitor.

EP 0 256 569 A1 discloses a mains supply unit that comprises alongitudinal controller and is controlled so that the voltage ripple onthe load moves towards zero, wherein the longitudinal controllertransistor only has to obliterate the minimum power loss possible, sinceonly the alternating voltage component is connected to said longitudinalcontroller transistor. In this case a load capacitor is also alwaysconnected in parallel with the load; no connectable buffer capacitorsare revealed.

SUMMARY OF THE INVENTION

It is the object of the invention to find a more convenient and lesscostly solution to problems that often occur in conjunction with buffercapacitors.

This object is achieved by means of a mains power supply unit whereinthe buffer capacitor can be charged by way of a dedicated charge circuitto a voltage, which, regardless of the voltage fluctuations in thedirect voltage branch, is substantially constant and corresponds insubstantial manner to the upper limit value of the direct voltage, towhich limit value the mains power supply unit is dimensioned and thebuffer capacitor can be connected to the direct voltage branch by way ofa controlled switch in the event of undervoltage in the mains supply orin the direct voltage branch.

The invention enables the buffer capacitor (or the buffer capacitors) toalways be charged to the maximum voltage, so that said capacitor storesa maximum quantity of energy and corresponds subsequently to a maximum,constant buffer time. It is therefore no longer necessary toover-dimension the capacitor. Since the buffer capacitor is only chargedon one occasion and is not discharged/charged in periodic manner, itsuse does not lead to increased harmonic content in an alternatingcurrent mains supply.

The supply voltage, which is, in general, an intermediate circuitvoltage, is thus connected directly to the consumer and/or the converterpreconnected thereto. The buffer capacitor is charged to its planned andpermissible maximum voltage by way of a charging circuit, which, forexample, can be a voltage multiplier, a straightforward flybackconverter or the like, wherein after the charging process only theleakage currents of the capacitor have to be compensated. When a failurein the mains supply is detected, the buffer capacitor (or a seriesand/or parallel connection of buffer capacitors) is connected with theaid of a suitable switch, in particular a semi-conductor switch, such asthyristor, triac, etc., to the consumer or intermediate circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention are characterized in the dependentsubordinate claims. The invention and its other advantages are explainedin detail below with reference to exemplified embodiments that areillustrated in the drawing, in which:

FIG. 1 shows a block circuit diagram of a first embodiment of theinvention,

FIG. 2 shows a block circuit diagram of second embodiment of theinvention, and

FIG. 3 shows a detailed circuit diagram of a practice-orientedembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a mains power supply unit in which a rectifier andinput part 1 is connected to three phases of an alternating voltagemains supply. The output of rectifier and input part 1, which cancontain filters, etc., in addition to rectifiers, is connected to anintermediate circuit direct voltage U1, which in turn is supplied to avoltage system converter 2. The output of voltage system converter 2 isconnected to one consumer or several consumers (not illustrated), forexample, computers, logical units, motors, etc. For example, the voltagesystem converter 2 can convert an intermediate circuit voltage of 320 Vinto a direct voltage of 24 V.

The intermediate circuit is provided with a buffer capacitor C, which,however, is not always connected to the intermediate circuit voltage U1,as is the case in the prior art. On the contrary, this buffer capacitorcan be connected in parallel with the intermediate circuit, that is, theinput of the voltage system converter 2 by way of a controlled circuitswitch S, and the buffer capacitor can be charged by way of a dedicatedcharging circuit 3 to a voltage U2 that is greater than the maximumvalue of the direct voltage U1 in the intermediate circuit. The voltageU2 can be obtained from the alternating current mains supply, forexample, by doubling the voltage, and can therefore be twice the valueof U1 (U2=2. U1). Of course, the charging circuit 3 can also be suppliedby the intermediate circuit voltage.

Disregarding unavoidable leakage current, it is evident that thecapacitor C is neither charged nor discharged during normal operation.

The switch S is controlled by a control circuit 4 that is arranged tocompare the intermediate circuit voltage U1 with a reference voltageU_(ref) and that controls the switch S in such a manner that said switchcloses, if the intermediate circuit voltage U1 falls below apredetermined value. However, it is also possible to compare the voltagein the alternating current mains supply to a reference value so as todetermine a mains supply failure or interruption.

When the mains supply voltage falls below a predetermined value, thebuffer capacitor C, which is charged to the voltage U2, is alsoconnected to the voltage system converter 2, which over a predeterminedbuffer time supplies the consumer with electric energy. It is known thatthis is necessary in critical applications, for example, in order tosafeguard data or to terminate a control procedure. A diode, forexample, must ensure that when connecting the capacitor C, the energythereof is not able to flow into the input part 1.

In general, the voltage at the capacitor C, regardless of the mainssupply voltage, should comprise a value that corresponds to the highestvalue beneficial for the buffer capacitor C and for the consumersconnected to the direct voltage branch.

As shown in FIG. 2, the direct voltage U1, for example, an intermediatecircuit voltage, is supplied, after division by a voltage divider R1/R2,to a Schmitt-trigger ST, the threshold voltage of which corresponds tothe reference voltage U_(ref) of FIG. 1. The Schmitt-trigger ST isfollowed by a monoflop MF. Downstream of this monoflop MF there isdisposed a control step SS for a thyristor switch S. The thyristorswitch S is connected in series with the buffer capacitor C, and a Zenerdiode DZ is connected in parallel therewith for the purpose of limitingthe voltage. The buffer capacitor C is charged by the charging circuit3, shown here as a current source.

One embodiment hitherto implemented in practice will now be explainedwith reference to FIG. 3. A three-phase alternating current mains supplyis provided from which a bridge rectifier V1 . . . V6 produces theintermediate circuit voltage U1, for example, 540 V. This voltage isconnected to the consumer, e.g., a converter, where appropriate, afterfiltering through capacitors and a choke (not illustrated).

In the present example, two electrolyte capacitors C5, C6 connected inseries are provided as buffer capacitors that in each case are to becharged to approximately 360 V. In this case a known voltage doublingcircuit consists of a capacitor C1 and the diodes V7 and V8, and chargesthe capacitors C5, C6 by way of a resistor R7 that limits the chargingcurrent. The maximum voltage provided at each capacitor C5 and/or C6 isfixed by virtue of a parallel-connected Zener diode V14 and V15,respectively, in the present example, to 360 V, respectively, so thatthe total voltage at C5+C6 amounts to 720 V.

A part of the intermediate circuit voltage U1 that is fixed by virtue ofa voltage divider R1/R2, is supplied to the inverting input of anoperational amplifier N1, which together with the resistors R4, R5, R6and R14, forms a Schmitt-trigger. The amplifier N1 is supplied by avoltage U_(Ref), in the present case, +5 Volts.

When the mains supply fails, the voltage U1 and therefore the voltage atthe voltage divider R1/R2 fall, at which point the output, connected byU_(Ref) by way of a pull-up-resistor R3, of the Schmitt-trigger jumpsfrom a predetermined value to positive level. This voltage jump passesby way of a diode V9 and a capacitor C2 to the non-inverting input of afurther operational amplifier N2, wherein the capacitor C2, togetherwith the resistor network R3, R11, R12, forms a differential element.

The inverting input of the amplifier N2, which is also supplied by thevoltage U_(Ref), is connected by way of the series connection of tworesistors R8 and R9 to an auxiliary voltage U_(H), in this case, +10Volts, and is connected by way of a capacitor C3 to earth. Theconnection point of the resistors R8 and R9 is linked by way of a diodeV10 to the output of the amplifier N2, which is connected to earth byway of the series connection of a resistor R11 and the resistor R12 andto the base of a Darlington-transistor pair V11+V12. The collectors ofthis pair are linked by way of a resistor R10 to the auxiliary voltageU^(H) and likewise by way of a capacitor C4 to earth. The emitter of thetransistor V12 is connected by way of a resistor R13 to the gate of aTRIAC-switch V13 whose switching path is between the negative terminalof the buffer capacitor series connection C5+C6 and earth. The TRIAC V13thus corresponds to the switch S of FIGS. 1 and 2.

As soon as the pulse described above passes by way of the capacitor C2to the non-inverting input of the amplifier N2, said amplifier isblocked and a corresponding voltage level of 2.5 Volts, for example, isforced by way of the voltage divider R9, R11, R12 at the non-invertinginput until the capacitor C3 has been charged to the same value by wayof the resistors R8 and R9. During this time a voltage pulse occurs atthe base of the Darlington pair V11/V12, for example, at rate of 4.7 V/27 ms, and the Darlington pair V11/V12 provides a current pulse to theTRIAC switch V13, which is blocked up to that point, the current pulseof which-for the purpose of discharging the source of the voltageU^(H)—is drawn mainly from the capacitor C4. During the entireon-transition time this current pulse is held by virtue of the currentsource V11, V12, R13 in a constant manner at the safe gate triggercurrent of the TRIAC, as required by the manufacturer, e.g., 100 mA. TheTRIAC switch V13 now switches through and applies the buffer capacitorseries connection C5 +C6 to the intermediate circuit for a predeterminedtime period. During this time the buffer capacitors can relay the mainpart of their energy content to the consumer, for example, a power of 1kW over 227 ms, in a typical case of application.

What is claimed is:
 1. Mains power supply unit for supplying consumersthat are connected directly or by way of a voltage system converter to adirect voltage branch of the mains power supply unit, comprising abuffer capacitor that can be charged from the mains supply and thatstill supplies the direct voltage branch for a predetermined minimumperiod of time in the event of a failure or interruption in the mainssupply; and dedicated charging circuit means for charging the buffercapacitor to a voltage that, regardless of the voltage fluctuations inthe direct voltage branch, is substantially constant and substantiallycorresponds to the upper limit value of the direct voltage, to whichlimit value the mains power supply unit is configured, and the buffercapacitor can be connected onto the direct voltage branch by way of acontrolled switch in the event of undervoltage in the mains supply or inthe direct voltage branch.
 2. Mains power supply unit according to claim1, characterized in that the control input of the controlled switch isconnected to the output of a control circuit, which is arranged tocompare the direct voltage of the direct voltage branch to a referencevoltage.
 3. Mains power supply unit according to claim 1 or 2, whereinthe mains supply is an alternating current mains supply, characterizedin that the charging circuit is a voltage doubling circuit connected tothe alternating voltage.
 4. Mains power supply unit according to any oneof the claims 1 to 3, characterized in that a voltage limiting circuitis provided for the purpose of limiting the capacitor voltage.
 5. Mainspower supply unit according to claim 4, characterized in that a Zenerdiode is connected in parallel with the buffer capacitor.